1. Field of the Invention
The present invention relates to digital signal transmission systems and particularly, to the Advanced Television Systems Committee (ATSC) Digital Television (DTV) standard (A/53). The invention describes a method for transmitting a robust bit-stream along with the standard bit-stream using the ATSC standard in a backward compatible manner.
2. Discussion of the Prior Art
The ATSC standard for high-definition television (HDTV) transmission over terrestrial broadcast channels uses a signal that comprises a sequence of twelve (12) independent time-multiplexed trellis-coded data streams modulated as an eight (8) level vestigial sideband (VSB) symbol stream with a rate of 10.76 MHz. This signal is converted to a six (6) MHz frequency band that corresponds to a standard VHF or UHF terrestrial television channel, over which the signal is broadcast at a data rate of 19.39 million bits per second (Mbps). Details regarding the (ATSC) Digital Television Standard and the latest revision A/53 is available at http://www.atsc.org/.
FIG. 1 is a block diagram generally illustrating an exemplary prior art high definition television (HDTV) transmitter 100. MPEG compatible data packets are first randomized in a data randomizer 105 and each packet is encoded for forward error correction (FEC) by a Reed Solomon (RS) encoder unit 110. The data packets in successive segments of each data field are then interleaved by data interleaver 120, and the interleaved data packets are then further interleaved and encoded by trellis encoder unit 130. Trellis encoder unit 130 produces a stream of data symbols having three (3) bits each. One of the three bits is pre-coded and the other two bits are produced by a four (4) state trellis encoder. The three (3) bits are then mapped to an 8-level symbol.
As known, a prior art trellis encoder unit 130 comprises twelve (12) parallel trellis encoder and pre-coder units to provide twelve interleaved coded data sequences. In multiplexer 140 the symbols of each trellis encoder unit are combined with “segment sync” and “field sync” synchronization bit sequences 150 from a synchronization unit (not shown). A small in-phase pilot signal is then inserted by pilot insertion unit 160 and optionally pre-equalized by filter device 165. The symbol stream is then subjected to vestigial sideband (VSB) suppressed carrier modulation by VSB modulator 170. The symbol stream is then finally up-converted to a radio frequency by radio frequency (RF) converter 180.
FIG. 2 is a block diagram illustrating an exemplary prior art high definition television (HDTV) receiver 200. The received RF signal is down-converted to an intermediate frequency (IF) by tuner 210. The signal is then filtered and converted to digital form by IF filter and detector 220. The detected signal is then in the form of a stream of data symbols that each signify a level in an eight (8) level constellation. The signal is then provided to NTSC rejection filter 230 and to synchronization unit 240. Then the signal is filtered in NTSC rejection filter 230 and subjected to equalization and phase tracking by equalizer and phase tracker 250. The recovered encoded data symbols are then subjected to trellis decoding by trellis decoder unit 260. The decoded data symbols are then further de-interleaved by data de-interleaver 270. The data symbols are then subjected to Reed Solomon decoding by Reed Solomon decoder 280. This recovers the MPEG compatible data packets transmitted by transmitter 100.
While the existing ATSC 8-VSB A/53 digital television standard is sufficiently capable of transmitting signals that overcome numerous channel impairments such as ghosts, noise bursts, signal fades and interferences in a terrestrial setting, there exists a need for flexibility in the ATSC standard so that streams of varying priority and data rates may be accommodated.